Determining Kinetic Barriers in Hydrogen Electrocatalysis in Alkaline Media through Single-Crystal Voltammetry and Kinetic Isotope Effects

Abstract

The sluggish hydrogen reaction kinetics observed in alkaline media compared to acidic media remain a fundamental challenge in modern electrocatalysis. Despite numerous efforts, the key parameters needed to accurately describe the slow hydrogen reaction kinetics at high pH are still unknown. Thorough understanding of the reaction mechanism and other relevant kinetic parameters is crucial for good electrocatalyst design needed for efficient energy conversion applications. In this work, we undertake an experimental and computational study to determine the effect of solvent dynamics in the kinetics of the hydrogen evolution and hydrogen oxidation reactions (HER and HOR). A possible explanation for the decreased reaction kinetics arises when considering the role of hydroxide on HER/HOR in alkaline media. The Markovic [1,2] group showed that HER/HOR activity can be significantly improved by decorating the catalyst surface with oxophilic sites (such as Ru or Ni(OH)2clusters on Pt), suggesting a “bifunctional” reaction pathway where hydroxide adsorbed on these oxophilic sites is an active participant in HER and HOR. Our past studies [3,4], however, indicate that a bifunctional mechanism where adsorbed hydrogen and hydroxide react is thermodynamically inconsistent, and that kinetic parameters must also be considered for explaining the sluggish HER and HOR kinetics in alkaline media. Other recent efforts have focused on the impact of such oxophilic clusters on the interfacial electric field and water network. The Koper and Feliu [5,6] groups showed that the introduction of Ni(OH)2clusters onto the Pt surface also decreases the ordering of the water network at the interface to accommodate charge transfer through the double layer, the energetics of which are controlled by how strongly water interacts with the interfacial electric field. The introduction of Ni(OH)2clusters therefore decreases the interfacial electric field strength and allows for a more dynamic water network near the interface. In another study, Lia et. al. [7] showed the importance of alkali cations for anchoring reactive species in the double layer region, in accordance with other reports by the Markovic and Koper groups [8,9]. While these studies highlight the profound impact that the interfacial water network rigidity and alkali cations have on the system, the fundamental role of solvent dynamics on adsorption processes and overall reaction kinetics is yet to be deciphered. In this work, we examine through the means of single-crystal voltammetry and microkinetic modeling the effect of solvent dynamics on the overall alkaline hydrogen reaction kinetics via a kinetic isotope effect. By comparing experimentally measured differences between H2O and D2O, we are able to probe the influence of interfacial water dynamics and electrolyte cation choice on adsorption processes (e.g. the Volmer step) and overall reaction kinetics. In addition, we focus on the reaction’s surface sensitivity by comparing Pt(111) and Pt(110) single crystals. Our preliminary results indicate that the HER and HOR kinetics are slower in D2O by approximately 20%, further showing that hydrogen and hydroxide binding energies are not the sole descriptors for HER and HOR in alkaline media, and that solvent reorganization kinetics are important limiting factors. Peak splitting measurements and comparisons with our microkinetic model suggest that the rate-determining step must be hydroxide adsorption, in agreement with the Koper and Feliu groups. The results presented herein reinforce the importance of local barriers and interfacial solvent dynamics for adsorption processes and overall reaction kinetics. [1]Subbaraman et. al. Nat. Mater. 2012, 550-557. [2] Strmcnik et. al. Nat. Chem. 2013, 5, 300–306. [3] Intikhab et. al. ACS Cat. 2017, 7, 8314−8319. [4] Rebollar et. al. JES, 2018, 165 (15), 3209-3221. [5] Ledezma et. al. Nat. Energy 2017, 2(4). [6] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623. [7] Lia et. al. J. Am. Chem. Soc., 2019, 141 (7), 3232–3239. [8] Subbaraman et. al. Science. 334 (2011) 334, 6060, 1256-1260. [9] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623.